Weldable high strength steel having excellent low temperature toughness

ABSTRACT

This invention adds elements such as Cu, B, Cr, Ca, V, etc., to a low carbon-high Mn--Ni--Mo-trace Ti type steel, and allows the steel to have a tempered martensite/bainite mixed structure containing at least 60% of tempered martensite transformed from un-recrystallized austenite having a mean austenite grain size (dγ) of not greater than 10 μm as a micro-structure, or a tempered martensite structure containing at least 90% of martensite transformed from un-recrystallized austenite. The present invention further stipulates a P value to the range of 1.9 to 4.0 and thus provides a ultra-high strength steel having a tensile strength of at least 950 MPa (not lower than 100 of the API standard) and excellent in low temperature toughness, HAZ toughness and field weldability in cold districts.

TECHNICAL FIELD

This invention relates to an ultra-high strength steel having a tensilestrength (TS) of at least 950 MPa and excellent in low temperaturetoughness and weldability, and this steel can widely be used for linepipes for transporting natural gases and crude oils and as a weldablesteel material for various pressure containers and industrial machinery.

BACKGROUND ART

Recently, the required strength of line pipes used for the long distancetransportation of crude oils and natural gases has become higher andhigher due to (1) an improvement in transportation efficiency by higherpressure, and (2) an improvement in laying efficiency due to thereduction of outer diameters and weights of line pipes. Line pipeshaving a strength of up to X80 according to the American PetroleumInstitute (API) (at least 620 MPa in terms of tensile strength) havebeen put into practical application in the past, but the need for linepipes having a higher strength has increased.

Conventionally, an ultra-low carbon-high Mn--Nb--(Mo)--(Ni)-traceB-trace Ti steel has been known as a line pipe steel having a structurecomprising mainly fine bainite, but the upper limit of its tensilestrength is at most 750 MPa. In this basic chemical composition system,an ultra-high strength steel having a structure mainly comprising finemartensite does not exist. It had been believed that a tensile strengthexceeding 950 MPa can never be attained by the structure mainlycomprising bainite and furthermore, the low temperature toughness isdeteriorated if the martensite structure increases.

Studies on the production method of ultra-high strength line pipes havebeen made at present on the basis of the conventional X80 line pipeproduction technologies (for example, "NKK Engineering Report", No. 138(1992), pp. 24-31, and "The 7th Offshore Mechanics and ArcticEngineering" (1998), Volume V, pp. 179-185), but the production of linepipes of X100 (tensile strength of at least 760 MPa) is believed to bethe limit according to these technologies.

To achieve an ultra-high strength in pipe lines, there are a largenumber of problems yet to be solved such as the balance of strength andlow temperature toughness, toughness of a welding heat affected zone(HAZ), field weldability, softening of a joint, and so forth, and anrapid development of a revolutionary ultra-high strength line pipe(exceeding X100) has been sought.

To satisfy the requirements described above, the present invention aimsat providing an ultra-high strength weldable steel having an excellentbalance between the strength and the low temperature toughness, beingeasily weldable on field and having a tensile strength of at least 950MPa (exceeding X100 of the API standard).

DISCLOSURE OF THE INVENTION

The inventors of the present invention have conducted intensive studieson the chemical components (compositions) of steel materials and theirmicro-structures in order to obtain an ultra-high strength steel havinga tensile strength of at least 950 MPa and excellent in low temperaturetoughness and field weldability, and have invented a new ultra-highstrength weldable steel.

It is the first object of the present invention to provide a newultra-high strength weldable steel, which is a low carbon-high Mn typesteel containing Ni--Mo--Nb-trace Ti compositely added thereto, andhaving a tensile strength of at least 950 MPa and excellent in lowtemperature toughness and site weldability in cold districts.

It is the second object of the present invention to provide a steelwhich has a P value, defined by the following chemical formula, withinthe range of 1.9 to 4.0 in the chemical compositions constituting theultra-high strength weldable steel described above. Needless to say,this P value changes somewhat depending on various ultra-high strengthweldable steels provided by the present invention.

The term "P value" (hardenability index) defined in the presentinvention represents a hardenability index. When this P value takes ahigh value, it indicates that the structure is likely to transform to amartensite or bainite structure. It is an index that can be used as astrength estimation formula of steels, and can be expressed by thefollowing general formula:

    P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+(1+β)MO+V-1+β

where β is 0 when B is less than 3 ppm and β is when B is greater thanor equal to 3 ppm.

It is the third object of the present invention to provide a weldablehigh strength steel excellent in low temperature toughness, wherein thechemical compositions constituting the ultra-high strength weldablesteel and the micro-structure of the steel have a specific structure,the micro-structure contains at least 60%, in terms of volume fraction,of martensite transformed from un-recrystallized austenite having anapparent mean austenite grain size (dγ) of not greater than 10 μm in asuitable combination with the chemical compositions constituting thesteel, and the sum of a martensite fraction and a bainite fraction is atleast 90%, or the micro-structure contains at least 60%, in terms ofvolume fraction, of martensite transformed from an un-recrystallizedaustenite having an apparent mean austenite grain size (dγ) of notgreater than 10 μm and the sum of a martensite fraction and a bainitefraction is at least 90%.

To achieve the objects described above, a weldable high strength steelhaving a low temperature toughness according to the present inventioncontains the following compositions, in terms of wt %:

C: 0.05 to 0.10%, Si≦0.6%,

Mn: 1.7 to 2.5%, P≦0.015%,

S:≦0.003%, Ni: 0.1 to 1.0%,

Mo: 0.15 to 0.60%, Nb: 0.01 to 0.10%,

Ti: 0.005 to 0.030%, Al: ≦0.06%, and

N: 0.001 to 0.006%.

The present invention provides a high strength steel containing thecomponents described above as the basic chemical compositions so as tosecure the required low temperature toughness and weldability. In orderto improve various required characteristics, particularly hardenability,the steel further contains 0.0003 to 0.0020% of B in addition to thebasic chemical compositions described above, and to improve the strengthand the low temperature toughness, the steel further contains 0.1 to1.2% of Cu. Furthermore, at least one of V: 0.01 to 0.10% and Cr: 0.1 to0.8% is added so as to refine the steel micro-structure, to increase thetoughness and to further improve the welding and HAZ characteristics.

At least one of Ca: 0.001 to 0.006%, REM: 0.001 to 0.02% and Mg: 0.001to 0.006% is added so as to control the shapes of inclusions such assulfides and to secure the low temperature toughness.

The terms "martensite" and "bainite" used herein represent not onlymartensite and bainite themselves but include so-called "temperedmartensite" and "tempered bainite" obtained by tempering them,respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the definition of an apparent mean austenite grain size(dγ).

BEST MODE FOR CARRYING OUT THE INVENTION

The first characterizing feature of the present invention resides inthat (1) the steel is a low carbon high Mn type (at least 1.7%) steel towhich Ni--Nb--Mo-trace Ti are compositely added, and (2) itsmicro-structure comprises fine martensite transformed from anun-recrystallized austenite having a mean austenite grain size (dγ) ofnot greater than 10 μm and bainite.

A low carbon-high Mn--Nb--Mo steel has been well known in the past as aline pipe steel having a fine acicular structure, but the upper limit ofits tensile strength is 750 MPa at the highest. In this basic chemicalcompositions, an ultra-high tension steel having a fine is temperedmartensite/bainite mixed structure does not exist. It has been believedthat a tensile strength higher than 950 MPa can never be attained in thetempered martensite/bainite structure of the Nb--Mo steel, and moreover,that the low temperature toughness and field weldability areinsufficient, too.

First, the micro-structure of the steel according to the presentinvention will be explained.

To accomplish a ultra-high strength of a tensile strength of at least950 MPa, the micro-structure of the steel material must comprise apredetermined amount of martensite, and its fraction must be at least60%. If the martensite fraction is not greater than 60%, a sufficientstrength cannot be obtained and moreover, it becomes difficult to securean excellent low temperature toughness (the most desirable martensitefraction for the strength and the low temperature toughness is 70 to90%). However, the intended strength/low temperature toughness cannot beaccomplished even when the martensite fraction is at least 60%, if theremaining structure is not suitable. Therefore, the sum of themartensite fraction and the bainite fraction must be at least 90%.

Even when the kind of micro-structure is limited as described above,excellent low temperature toughness cannot always be obtained. To obtainexcellent low temperature toughness, it is necessary to optimize theaustenite structure before the γ-to-α transformation (prior austenitestructure), and to effectively refine the final structure of the steelmaterial. For this reason, the present invention limits the prioraustenite structure to the un-recrystallized austenite and its meangrain size (dγ) to not greater than 10 μm. It has been found that anexcellent balance of strength and low temperature toughness can beobtained even in the mixed structure of martensite and bainite in theNb--Mo steel whose low temperature toughness has been believed inferiorin the past, by such limitations.

The reduction of the un-recrystallized austenite grain size into a finegrain size is particularly effective for improving the low temperaturetoughness of the Nb--Mo type steel according to the present invention.To obtain the intended low temperature toughness (for example, nothigher than -80° C. by a transition temperature of a V-notch Charpyimpact test), the mean grain size must be smaller than 10 μm. Here, theapparent mean austenite grain size is defined as shown in FIG. 1, and adeformation band and a twin boundary having similar functions to thoseof the austenite grain boundary are included in the measurement of theaustenite grain size. More concretely, the full length of the straightline drawn in the direction of thickness of a steel plate is divided bythe number of points of intersection with the austenite grain boundaryexisting of this straight line to determine dγ. It has been found outthat the austenite mean grain size so determined has an extremely closecorrelation with the low temperature toughness (transition temperatureof the Charpy impact test).

It has been also clarified that when the chemical compositions (additionof high Mn--Nb-high Mo) of the steel material and its micro-structure(un-recrystallization of austenite) are strictly controlled as describedabove, a separation occurs on the fracture of the Charpy impact test,etc., and a fracture area transition temperature can be furtherimproved. The separation is a laminar peel phenomenon occurring on thefracture of the Charpy impact test etc., parallel to the plate surface,and is believed to lower the degree of a triaxial stress at a brittlecrack tip and to improve brittle crack propagation stoppingcharacteristics.

The second characterizing feature of the present invention is that (1)the steel is a low carbon-high Mn type steel to which Ni--Mo--Nb-traceB-trace Ti are compositely added, and (2) and its micro-structure mainlycomprises a fine martensite structure transformed from un-recrystallizedaustenite having a mean austenite grain size (dγ) of not greater than 10μm.

The third characterizing feature of the present invention is that (1)the steel is a low carbon high Mn type (at least 1.7%) Cu precipitationhardening steel which contains 0.8 to 1.2% of Cu and to whichNi--Nb--Cu--Mo-trace Ti are compositely added, and (2) itsmicro-structure comprises fine martensite and bainite transformed fromun-recrystallized austenite having a mean austenite grain size of notgreater than 10 μm.

Cu precipitation hardening type steels have been used in the past forhigh strength steels (tensile strength of a 784 MPa class) for pressurecontainers, but no example of development in an ultra-high strength linepipe of higher than X100 has been found. This is presumably because theCu precipitation hardening steel can easily obtain the strength but itslow temperature toughness is not sufficient for the line pipe.

As to the low temperature toughness, propagation stoppingcharacteristics are extremely important together with the occurrencecharacteristics of brittle rupture in the pipe lines. In theconventional Cu precipitation hardening steel, the occurrencecharacteristics of the brittle rupture typified by the Charpycharacteristics are considerably satisfactory, but the stopcharacteristics of the brittle rupture are not sufficient. For, (1)refining of the micro-structure is not sufficient, and (2) the so-called"separation" occurring on the fracture of Charpy impact test is notutilized. (This separation is a laminar peel phenomenon occurring on thefracture of the Charpy impact test, etc., parallel to the plate surface,and is believed to lower the degree of the triaxial stress at the distalend of the brittle crack and to improve the brittle crack propagationstopping characteristics).

However, even when the kind of the micro-structure is limited asdescribed above, a satisfactory low temperature toughness cannot alwaysbe obtained. To obtain the excellent low temperature toughness, it isnecessary to optimize the austenite structure before the γ-to-αtransformation and to effectively refine the final structure of thesteel material. Therefore, the present invention limits the prioraustenite structure to the un-recrystallized austenite and its meangrain size (dγ) to not greater than 10 μm. It has been found out in thisway that an extremely excellent balance of the strength and the lowtemperature toughness can be obtained even in the mixed structure ofmartensite and bainite of the Nb-Cu steel whose low temperaturetoughness had been believed to be inferior in the past.

Refining of the un-recrystallized austenite grain size is particularlyeffective for improving the low temperature toughness of the Nb-Cu typesteel of the present invention. To obtain the intended low temperaturetoughness (a transition temperature of not higher than -80° C. in theV-notch Charpy impact test), the mean grain size must be smaller than 10μm. Here, the apparent mean austenite grain size is defined as shown inFIG. 1, and the transformation band and the twin boundary having thesimilar functions to those of the austenite grain boundary are includedin the measurement of the austenite grain size. More concretely, thefull length of the straight line drawn in the direction of thickness ofthe steel plate is divided by the number of intersections with theaustenite grain boundary existing on the straight line to determine dγ.It has been found out that the mean austenite grain size determined inthis way has an extremely close correlationship with the low temperaturetoughness (transition temperature of the Charpy impact test).

It has been also clarified that when the chemical compositions of thesteel material (addition of high Mn--Nb--Mo--Cu) and the form of themicro-structure (un-recrystallization of austenite) are strictlycontrolled as described above, the separation occurs on the fracture ofthe Charpy impact test, etc., and the fracture transition temperaturecan be further improved.

To accomplish an ultra-high strength of a tensile strength of at least950 MPa, the micro-structure of the steel must comprise a predeterminedamount of martensite, and its fraction must be at least 90%. If themartensite fraction is smaller than 90%, a sufficient strength cannot beobtained, and moreover, it becomes difficult to secure a satisfactorylow temperature toughness.

However, even when the micro-structure of the steel is strictlycontrolled as described above, the steel material having the intendedcharacteristics cannot be obtained. To accomplish this object, thechemical compositions must be limited simultaneously with themicro-structure.

Hereinafter, the reasons for limitation of the chemical compositionalelements will be explained.

The C content is limited to 0.05 to 0.10%. Carbon is extremely effectivefor improving the strength of the steel, and at least 0.05% of C isnecessary so as to obtain the target strength in the martensitestructure. If the C content is too great, however, the low temperaturetoughness of both the base metal and the HAZ and field weldability areremarkably deteriorated. Therefore, the upper limit of C is set to0.10%. Preferably, however, the upper limit value is limited to 0.08%.

Si is added for deoxidation and for improving the strength. If itsaddition amount is too great, however, the HAZ toughness and fieldweldability are remarkably deteriorated. Therefore, its upper limit isset to 0.6%. Deoxidation of the steel can be attained sufficiently by Alor Ti, and Si need not always be added.

Mn is an indispensable element for converting the micro-structure of thesteel of the present invention to a structure mainly comprisingmartensite and for securing the excellent balance between strength andlow temperature toughness, and its lower limit is 1.7%. If the additionamount of Mn is too high, however, hardenability of the steel increases,so that not only the HAZ toughness and field weldability aredeteriorated, but center segregation of a continuous cast slab ispromoted and the low temperature toughness of the base metal isdeteriorated, too. Therefore, the upper limit is set to 2.5%.

The object of addition of Ni is to improve the low carbon steel of thepresent invention without deteriorating the low temperature toughnessand field weldability. In comparison with the addition of Cr and Mo, theaddition of Ni results in less formation of the hardened structure inthe rolled structure (particularly, the center segregation band of thecontinuous cast slab), which is detrimental to the low temperaturetoughness, and it has been found out further that the addition of asmall amount of Ni of at least 0.1% is effective for improving the HAZtoughness, too. (From the aspect of the HAZ toughness, a particularlyeffective amount of addition of Ni is at least 0.3%). If the additionamount is too high, however, not only economy but also the HAZ toughnessand field weldability are deteriorated. Therefore, its upper limit isset to 1.0%. The addition of Ni is also effective for preventing the Cucrack during continuous casting and during hot rolling. In this case, Nimust be added in an amount at least 1/3 of the Cu amount.

Mo is added so as to improve hardenability of the steel and to obtainthe intended structure mainly comprising martensite. In B-containingsteels, a effect of Mo on the hardenability increases, and the multipleof Mo in the later-appearing P value becomes 2 in the B steel incomparison with 1 in the B-free steel. Therefore, the addition of Mo isparticularly effective in the B-containing steels. When co-present withNb, Mo supresses recrystallization of austenite during controlledrolling, and is also effective for refining the austenite structure. Toobtain such effects, at least 0.15% of Mo is necessary. However, theaddition of Mo in an excessive amount causes deterioration of the HAZtoughness and field weldability and furthermore, extinguishes thehardenability improving effect of B. Therefore, its upper limit is setto 0.6%.

Further, the steel according to the present invention contains 0.01 to0.10% of Nb and 0.005 to 0.030% of Ti as the indispensable elements.When co-present with Mo, Nb not only surpresses recrystallization ofaustenite during controlled rolling to thereby refine the structure, butmakes a great contribution to precipitation hardening and the increaseof hardenability, and makes the steel tougher. Particularly when Nb andB are co-present, the hardenability improvement effect can be increasedsynergistically. However, if the addition amount of Nb is too high, theHAZ toughness and field weldability are adversely affected. Therefore,its upper limit is set to 0.10%. On the other hand, the addition of Tiforms TiN, supresses coarsening of the austenite grain during reheatingand the austenite grains of the HAZ, refines the micro-structure andimproves the low temperature toughness of both the base metal and theHAZ, It also has the function of fixing solid solution N, which isdetrimental to the hardenability improvement effect of B, as TiN. Forthis purpose, at least 3.4N (wt %) of Ti is preferably added. When theAl content is small (such as not greater than 0.005%), Ti forms anoxide, functions as an intra-grain ferrite formation nucleus in the HAZ,and refines the HAZ structure. In order to cause TiN to exhibit sucheffects, at least 0.005% of Ti must be added. If the Ti content is toohigh, coarsening of TiN and precipitation hardening due to TiC occur andthe low temperature toughness gets deteriorated. Therefore, its upperlimit is set to 0.03%.

Al is ordinarily contained as a deoxidation agent in the steel, and hasalso the effect of refining the structure. If the Al content exceeds0.06%, however, alumina type nonmetallic inclusions increase and spoilthe cleanness of the steel. Therefore, its upper limit is set to 0.06%.Deoxidation can be accomplished by Ti or Si, and Al need not be alwaysadded.

N forms TiN, supresses coarsening of the austenite grains duringreheating of the slab and the austenite grains of the HAZ, and improvesthe low temperature toughness of both the base metal and the HAZ, Theminimum necessary amount for this purpose is 0.001%. If the N content istoo high, however, N results in surface defects on the slab,deterioration of the HAZ toughness and a drop in the hardenabilityimprovement effect of B. Therefore, its upper limit must be limited to0.006%.

In the present invention, the P and S content as the impurity elementsare set to 0.015% and 0.003%, respectively. The main reason is tofurther improve the low temperature toughness of both the base metal andthe HAZ. The reduction of the P content reduces center segregation ofthe continuous cast slab, prevents the grain boundary cracking andimproves the low temperature toughness. The reduction of the S contentreduces MnS, which is elongated by hot rolling, and improves theductility and the toughness.

Next, the object of the addition of B, Cu, Cr and V will be explained.

The main object of the addition of these elements besides the basicchemical compositions is to further improve the strength and thetoughness and to enlarge the sizes of steel materials that can beproduced, without spoiling the excellent features of the presentinvention. Therefore, the addition amounts of these elements should benaturally limited.

An extremely small amount of B drastically improves hardenability of thesteel. Therefore, B is an essentially indispensable element in the steelof the present invention. It has an effect corresponding to a value 1 inthe later-appearing P value, that is, 1% Mn. Further, B enhances thehardenability improvement effect of Mo, and synergistically improveshardenability when copresent with Nb. To obtain such effects, at least0.0003% of B is necessary. When added in an excessive amount, on theother hand, B not only deteriorates the low temperature toughness butextinguishes, in some cases, the hardenability improvement effect of B.Therefore, its upper limit is set to 0.0020%.

The object of the addition of Cu is to improve the strength of the lowcarbon steel of the present invention without deteriorating the lowtemperature toughness. When compared with the addition of Mn, Cr and Mo,the addition of Cu does not form a hardened structure, which isdetrimental to the low temperature toughness, in the rolled structure(particularly, in the center segregation band of the slab), and is foundto increase the strength. When added in an excessive amount, however, Cudeteriorates field weldability and the HAZ toughness. Therefore, itsupper limit is set to 1.2%.

Cu increases the strength of both the base metal and the weld portion,but when its addition amount is too high, the HAZ toughness and fieldweldability are remarkably deteriorated. Therefore, the upper limit ofthe Cr content is 0.8%.

V has substantially the same effect as Nb, but its effect is weaker thanthat of Nb. However, the effect of the addition of V in the ultra-highstrength steel is high, and the composite addition of Nb and V makes theexcellent features of the steel of the present invention all the moreremarkable. The addition amount of up to 0.10% is permissible from theaspect of the HAZ toughness and field weldability, and a particularlypreferred range of the addition amount is from 0.03 to 0.08%.

Further, the object of the addition of Ca, REM and Mg will be explained.

Ca and REM control the form of the sulfide (MnS) and improve the lowtemperature toughness (the increase of absorption energy in the Charpytest, etc.). If the Ca or REM content is not greater than 0.001%,however, no practical effect can be obtained, and if the Ca contentexceeds 0.006% or if the REM content exceeds 0.02%, large quantities ofCaO--CaS or REM--CaS are formed and are converted to large clusters andlarge inclusions, and they not only spoil cleanness of the steel butalso exert adverse influences on field weldability. Therefore, the upperlimit of the Ca addition amount is limited to 0.006% or the upper limitof the REM addition amount is limited to 0.02%. By the way, it isparticularly effective in ultra-high strength line pipes to reduce the Sand O contents to 0.001% and 0.002%, respectively, and to set therelation ESSP=(Ca) 1-124(O)!/1.25S to 0.5≦ESSP≦10.0.

Mg forms a finely dispersed oxide, supresses coarsening of the grains atthe welding heat affected zone and improves the toughness. If the amountof addition is less than 0.001%, the improvement of the toughness cannotbe observed, and if it exceeds 0.006%, coarse oxides are formed, and thetoughness is deteriorated.

In addition to the limitation of the individual addition elementsdescribed above, the present invention limits the afore-mentioned Pvalue, that is, P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+(1-β)Mo+V-1+β, to1.9≦P≦4. By the way, β takes a value 0 when B<3 ppm and a value 1 whenB≧3 ppm. This is to accomplish the intended balance between the strengthand the low temperature toughness. The reason why the lower limit of theP value is set to 1.9 is to obtain a strength of at least 950 MPa and anexcellent low temperature toughness. The upper limit of the P value islimited to 4.0 in order to maintain the excellent HAZ toughness andfield weldability.

When the high strength steel having excellent low temperature toughnessaccording to the present invention is produced, the following productionmethod is preferably employed.

After a steel slab having the chemical compositions of the presentinvention is reheated to a temperature within the range of 950° to1,300° C., the slab is hot rolled so that a cumulative rolling reductionamount at a temperature not higher than 950° C. is at least 50% and ahot rolling finish temperature is not lower than 800° C. Next, coolingis carried out at a cooling rate of at least 10° C./sec down to anarbitrary temperature below 500° C. Tempering is carried out, whenevernecessary, at a temperature below an Ac₁ point.

The lower limit of the reheating temperature of the steel slab isdetermined so that solid solution of the elements can be accomplishedsufficiently, and the upper limit is determined by the condition underwhich coarsening of the crystal grains does not become remarkable.

The temperature below 950° C. represents an un-recrystallizationtemperature zone, and in order to obtain the intended fine grain size, acumulative rolling reduction quantity of at least 50% is necessary. Thefinish hot-rolling temperature is limited to not lower than 800° C. atwhich bainite is not formed. Thereafter, cooling is carried out at acooling rate of at least 10° C./sec so as to form the martensite andbainite structure. Since transformation finishes substantially at 500°C., cooling is made to a temperature below 500° C.

Furthermore, tempering treatment can be carried out in the steel of thepresent invention at a temperature below the Ac₁ point. This temperingtreatment can suitably recover the ductility and the toughness. Thetempering treatment does not change the micro-structure fraction itself,does not spoil the excellent features of the present invention and hasthe effect of narrowing the softening width of the welding heat affectedzone.

Next, Examples of the present invention will be described.

EXAMPLE 1

Slabs having various chemical compositions were produced by melting on alaboratory scale (50 kg, 120 mm-thick ingot) or a convertercontinuous-casting method (240 mm-thick). These slabs were hot-rolledinto steel plates having a thickness of 15 to 28 mm under variousconditions. The mechanical properties of each of the steel plates sorolled and its micro-structure, were examined.

The mechanical properties (yield strength: YS, tensile strength: TS,absorption energy at -40° C. in the Charpy impact test: vE₋₄₀ andtransition temperature: vTrs) of the steel plates were measured in adirection orthogonal to the rolling direction. The HAZ toughness(absorption energy at -20° C. in the Charpy impact test: vE₋₂₀) wasevaluated by the simulated HAZ specimens (maximum heating temperature:1,400° C., cooling time from 800° to 500° C.: .increment.t₈₀₀₋₅₀₀ !: 25seconds). Field weldability was evaluated as the lowest preheatingtemperature necessary for preventing the low temperature cracks of theHAZ by the y-slit weld crack test (JIS G3158) (welding method: gas metalarc welding, welding rod: tensile strength of 100 MPa, heat input: 0.5kJ/mm, hydrogen content of welding metal: 3 cc/100 g).

Tables 1 and 2 show the Examples. The steel plates produced inaccordance with the present invention had the excellent balance of thestrength and the low temperature toughness, the HAZ toughness and fieldweldability. In contrast, Comparative Examples were remarkably inferiorin their characteristics because the chemical compositions or theirmicro-structures were not suitable.

Because the C content was too great in Steel No. 9, the Charpy absorbedenergy of the base metal and the HAZ was low, and the preheatingtemperature at the time of welding was also high. Because Ni was notadded in Steel No. 10, the low temperature toughness of the base metaland the HAZ was inferior. Because the Mn addition amount and the P valuewere too great in Steel No. 11, the low temperature toughness of thebase metal and the HAZ was inferior, and the preheating temperature atthe time of welding was also extremely high.

Because Nb was not added in Steel No. 12, the strength was insufficient,the austenite grain size was large, and the toughness of the base metalwas inferior.

                                      TABLE 1                                     __________________________________________________________________________                                                                steel                                                                         plate                      chemical compositions (wt %, *ppm)                 thick-                                                                    P   ness              section                                                                             steel                                                                            C   Si Mn  P* S* Ni  Mo Nb  Ti  Al N*  others  value                                                                             (mm)              __________________________________________________________________________    steel 1  0.058                                                                             0.26                                                                             2.37                                                                              100                                                                              15 0.40                                                                              0.43                                                                             0.041                                                                             0.009                                                                             0.027                                                                            23          2.24                                                                              15                of this                                                                             2  0.093                                                                             0.32                                                                             1.89                                                                              60 8  0.48                                                                              0.57                                                                             0.024                                                                             0.012                                                                             0.018                                                                            40  Mg:0.002                                                                              1.96                                                                              20                invention                                                                           3  0.064                                                                             0.18                                                                             2.15                                                                              70 3  0.24                                                                              0.38                                                                             0.017                                                                             0.021                                                                             0.024                                                                            56  Cr:0.34 2.16                                                                              20                      4  0.070                                                                             0.27                                                                             2.10                                                                              50 7  0.34                                                                              0.51                                                                             0.038                                                                             0.015                                                                             0.027                                                                            38  Cu:0.39 2.24                                                                              20                      5  0.073                                                                             0.23                                                                             2.24                                                                              120                                                                              18 0.18                                                                              0.46                                                                             0.041                                                                             0.016                                                                             0.034                                                                            27  V:0.05  2.12                                                                              20                      6  0.067                                                                             0.02                                                                             2.13                                                                              80 6  0.36                                                                              0.47                                                                             0.132                                                                             0.015                                                                             0.019                                                                            37  V:0.06,                                                                               2.20.41                                                                           20                      7  0.075                                                                             0.27                                                                             2.01                                                                              60 10 0.35                                                                              0.45                                                                             0.038                                                                             0.016                                                                             0.002                                                                            33  V:0.07,                                                                               2.54.37                                                                           22                                                                Cr:0.58                             8  0.072                                                                             0.12                                                                             2.03                                                                              70 5  0.52                                                                              0.43                                                                             0.038                                                                             0.017                                                                             0.028                                                                            35  V:0.07,                                                                               2.24.53                                                                           28                                                                Ca:0.0021                     Compar-                                                                             9  0.117                                                                             0.26                                                                             2.01                                                                              80 15 0.37                                                                              0.38                                                                             0.032                                                                             0.015                                                                             0.021                                                                            29          1.98                                                                              15                ative 10 0.076                                                                             0.21                                                                             2.16                                                                              50 7  --  0.46                                                                             0.046                                                                             0.014                                                                             0.031                                                                            36  Cu:0.32 2.05                                                                              20                Steels                                                                              11 0.079                                                                             0.28                                                                             2.62                                                                              60 5  0.38                                                                              0.42                                                                             0.039                                                                             0.015                                                                             0.028                                                                            42  Cr:0.38 2.84                                                                              20                      12 0.072                                                                             0.27                                                                             2.08                                                                              70 5  0.37                                                                              0.46                                                                             0.004                                                                             0.018                                                                             0.025                                                                            29          2.01                                                                              20                __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                 micro-structure                  HAZ                                      temper-                                                                           austenite                                                                            mar- martensite/          tough-                                                                             field weldability                   ing mean grain                                                                           tensite                                                                            bainite                                                                             mechanical properties                                                                        ness lowest preheating                   treat-                                                                            size   fraction                                                                           fraction                                                                            YS  TS vE.sub.-40                                                                        vTrs                                                                              vE.sub.-20                                                                         temperature                section                                                                            steel                                                                             ment                                                                              (μm)                                                                              (%)  (%)   (N/mm.sup.2)                                                                         (J) (°C.)                                                                      (J)  (°C.)               __________________________________________________________________________    steel of                                                                           1   ∘                                                                     5.3    97   100   892 1025                                                                             234 -100                                                                              213  preheating not                                                                necessary                  this 1'  x   5.3    97   100   845 1081                                                                             211 -95 213  preheating not                                                                necessary                  inven-                                                                             2   ∘                                                                     7.6    79   97    918 1076                                                                             208 -85 187  preheating not                                                                necessary                  tion 3   ∘                                                                     8.2    94   100   872  978                                                                             217 -95 159  preheating not                                                                necessary                       3'  x   8.2    79   97    863 1122                                                                             195 -80 187  preheating not                                                                necessary                       4   ∘                                                                     7.3    96   100   869  981                                                                             302 -120                                                                              202  preheating not                                                                necessary                       5   ∘                                                                     7.1    91   100   903 1018                                                                             231 -110                                                                              167  preheating not                                                                necessary                       6   ∘                                                                     6.7    89   100   884  979                                                                             302 -110                                                                              320  preheating not                                                                necessary                       7   ∘                                                                     7.4    83   100   874  984                                                                             276 -105                                                                              307  preheating not                                                                necessary                       7'  x   7.4    83   100   821 1030                                                                             265 -95 307  preheating not                                                                necessary                       8   ∘                                                                     8.9    75   100   862  970                                                                             285 -110                                                                              243  preheating not                                                                necessary                  Compara-                                                                           9       6.9    89   100   926 1098                                                                             124 -80 56   100                        tive 10      7.2    93   100   856  973                                                                             78  -55 73   preheating not                                                                necessary                  Steels                                                                             11      6.6    100  100   967 1127                                                                             34  -60 20   150                             12      12.8   87   93    798  894                                                                             37  -50 256  preheating not                                                                necessary                  __________________________________________________________________________

EXAMPLE 2

Slabs having various chemical compositions components were produced bymelting on a laboratory scale (50 kg, 100 mm-thick ingots) or by aconverter-continuous casting method (240 mm-thick). These slabs werehot-rolled to steel plates having a plate thickness of 15 to 25 mm undervarious conditions. Various properties of the steel plates so rolled andtheir micro-structures were examined. The mechanical properties (yieldstrength: YS, tensile strength: TS, absorption energy at -40° C. in theCharpy test: vE₋₄₀, and 50% fracture transition temperature: vTrs) wereexamined in a direction orthogonal to the rolling direction. The HAZtoughness (absorption energy at -40° C. in the Charpy test: vE₋₄₀) wasevaluated by the simulated HAZ specimens (maximum heating temperature:1,400° C., cooling time from 800° to 500° C. .increment.t₈₀₀₋₅₀₀ !: 25seconds). Field weldability was evaluated by the lowest preheatingtemperature necessary for preventing the low temperature crack of theHAZ in the y-slit weld crack test (JIS G3158) (welding method: gas metalarc welding, welding rod: tensile strength of 100 MPa, heat input: 0.3kJ/mm, hydrogen amount of weld metal: 3 cc/100 g metal).

Tables 1 and 2 show the Examples. The steel plates produced inaccordance with the method of the present invention exhibited theexcellent balance between the strength and the low temperaturetoughness, the HAZ toughness and field weldability. In contrast,Comparative Steels were obviously and remarkably inferior in any oftheir characteristics because the chemical compositions or themicro-structures were not suitable.

EXAMPLE 3

Slabs having various chemical compositions were produced by melting on alaboratory scale (50 kg, 120 mm-thick) or a converter-continuous castingmethod (240 mm-thick). These slabs were hot-rolled to steel plateshaving a plate thickness of 15 to 30 mm under various conditions.Various properties of the steel plates so rolled and theirmicro-structures were examined.

The mechanical properties (yield strength: YS, tensile strength: TS,absorption energy at -40° C. in the Charpy impact test: vE₋₄₀ andtransition temperature: vTrs) were examined in a direction rothogonal tothe rolling direction.

The HAZ toughness (absorption energy at -20° C. in the Charpy impacttest: vE₋₂₀) was evaluated by the simulated HAZ specimens (maximumheating temperature: 1,400° C., cooling time from 800° to 500° C..increment.t₈₀₀₋₅₀₀ !: 25 seconds).

Field weldability was evaluated by the lowest preheating temperaturenecessary for preventing the low temperature crack of the HAZ in they-slit weld crack test (JIS G3158) (welding method: gas metal arcwelding, welding rod: tensile strength of 100 MPa, heat input: 0.5kJ/mm, hydrogen amount of weld metal: 3 cc/100 g).

Examples are shown in Tables 1 and 2. The steel plates produced inaccordance with the present invention exhibited the excellent balance ofthe strength and the toughness, the HAZ toughness and field weldability.In contrast, Comparative Steels were remarkably inferior in any of theircharacteristics because the chemical compositions or themicro-structures were not suitable.

Because the C content was too high in Steel No. 9, Charpy absorptionenergy of the base metal and the HAZ was low, and the preheatingtemperature at the time of welding was high, too. Because the Mn and Pcontents were too high in Steel No. 10, the low temperature of both thebase metal and the HAZ was inferior, and the preheating temperature atthe time of welding was high, too.

Because the S content was too high in Steel No. 11, absorption energy ofthe base metal and the HAZ was low.

INDUSTRIAL APPLICABILITY

According to the present invention, it becomes possible to stablyproduce large quantities of steels for an ultra-high strength line pipes(tensile strength of at least 950 MPa and exceeding X100 of the APIstandard) having excellent low temperature toughness and fieldweldability. As a result, safety of the piplines can be remarkablyimproved, and transportation efficiency of the pipelines and executionefficiency can be drastically improved.

                                      TABLE 3                                     __________________________________________________________________________    chemical compositions of steels (wt %)                                        sec-                                             P                            tion                                                                             steel                                                                            C  Si Mn P  S  Ni Mo Nb T  B   Al N   others                                                                             value                        __________________________________________________________________________    steel                                                                            1  0.06                                                                             0.24                                                                             1.95                                                                             0.003                                                                            0.001                                                                            0.36                                                                             0.35                                                                             0.031                                                                            0.012                                                                            0.0007                                                                            0.024                                                                            0.0027   3.07                         of 2  0.07                                                                             0.05                                                                             1.76                                                                             0.012                                                                            0.002                                                                            0.78                                                                             0.35                                                                             0.015                                                                            0.015                                                                            0.0012                                                                            0.006                                                                            0.0035                                                                            Cu: 0.60                                                                           3.29                         this                                                                             3  0.05                                                                             0.31                                                                             2.12                                                                             0.009                                                                            0.002                                                                            0.81                                                                             0.24                                                                             0.035                                                                            0.017                                                                            0.0010                                                                            0.006                                                                            0.0041                                                                            Cr: 0.5                                                                            3.62                         in-                                                                              4  0.08                                                                             0.17                                                                             2.02                                                                             0.014                                                                            0.001                                                                            0.45                                                                             0.45                                                                             0.018                                                                            0.013                                                                            0.0005                                                                            0.038                                                                            0.0027                                                                            V: 0.06                                                                            3.41                         ven-                                                                             5  0.06                                                                             0.40                                                                             2.13                                                                             0.006                                                                            0.003                                                                            0.25                                                                             0.38                                                                             0.024                                                                            0.021                                                                            0.0015                                                                            0.019                                                                            0.0022                                                                            Ca: 0.004                                                                          3.32                         tion                                                                             6  0.06                                                                             0.23                                                                             2.17                                                                             0.008                                                                            0.001                                                                            0.37                                                                             0.21                                                                             0.032                                                                            0.012                                                                            0.0009                                                                            0.045                                                                            0.0048   3.01                            7  0.07                                                                             0.01                                                                             1.87                                                                             0.012                                                                            0.002                                                                            0.60                                                                             0.20                                                                             0.027                                                                            0.014                                                                            0.0013                                                                            0.011                                                                            0.0029                                                                            Cr: 0.3,                                                                           3.11                                                                     Cu: 0.3                              8  0.09                                                                             0.26                                                                             1.96                                                                             0.005                                                                            0.001                                                                            0.37                                                                             0.33                                                                             0.030                                                                            0.018                                                                            0.0008                                                                            0.033                                                                            0.0021   3.13                         Com-                                                                             9  0.07                                                                             0.28                                                                             1.94                                                                             0.004                                                                            0.002                                                                            0.40                                                                             0.38                                                                             0.033                                                                            0.012                                                                            0.0030                                                                            0.029                                                                            0.0035   3.18                         para-                                                                            10 0.06                                                                             0.25                                                                             1.96                                                                             0.008                                                                            0.001                                                                            0.21                                                                             0.75                                                                             0.036                                                                            0.013                                                                            0.0014                                                                            0.030                                                                            0.0032   3.82                         tive                                                                             11 0.06                                                                             0.18                                                                             1.60                                                                             0.010                                                                            0.001                                                                            0.38                                                                             0.22                                                                             0.037                                                                            0.020                                                                            0.0011                                                                            0.043                                                                            0.0035                                                                            Cu: 0.4                                                                            2.63                         Steels                                                                           12 0.08                                                                             0.31                                                                             2.53                                                                             0.008                                                                            0.001                                                                            0.86                                                                             0.32                                                                             0.035                                                                            0.024                                                                            0.0013                                                                            0.035                                                                            0.0034   3.90                         __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________                                                 field                                              micro-                 HAZ weldability                                        structure                                                                             mechanical     tough-                                                                            lowest                           plate                martensite                                                                         properties     ness                                                                              preheating                               thickness                                                                          tempering                                                                          dy ratio                                                                              YS  TS  vE.sub.-40                                                                       vTrs                                                                              vE.sub.-20                                                                        temp.                            section                                                                            steel                                                                            (mm) °C. × min.                                                            (μm)                                                                          (%)  (MPa)                                                                             (MPa)                                                                             (J)                                                                              (°C.)                                                                      (J) (°C.)                     __________________________________________________________________________    steel of                                                                           1  20   --   7.3                                                                              97   831 1163                                                                              204                                                                              -100                                                                              175 preheating                       this                                         not necessary                    invention                                                                          1  20   550  7.3                                                                              97   966  993                                                                              218                                                                              -120                                                                              176 preheating                                                                    not necessary                         2  20   --   5.1                                                                              95   835 1147                                                                              205                                                                              -110                                                                              174 preheating                                                                    not necessary                         3  25   550  8.5                                                                              92   903 1002                                                                              221                                                                              -95 198 preheating                                                                    not necessary                         4  25   550  7.9                                                                              92   878  995                                                                              204                                                                              -100                                                                              168 preheating                                                                    not necessary                         5  20   --   6.6                                                                              94   855 1171                                                                              205                                                                              105 173 preheating                                                                    not necessary                         6  16   --   5.4                                                                              98   819 1158                                                                              207                                                                              -130                                                                              184 preheating                                                                    not necessary                         6  16   550  5.4                                                                              98   877 1110                                                                              206                                                                              -95 187 preheating                                                                    not necessary                         7  20   --   7.8                                                                              93   842 1135                                                                              223                                                                              -95 179 preheating                                                                    not necessary                         8  20   550  8.2                                                                              91   1001                                                                              1089                                                                              186                                                                              85  158 preheating                                                                    not necessary                    Compara-                                                                           6  20   --   14.6                                                                             96   799 1162                                                                              210                                                                              -65 183 preheating                       tive                                         not necessary                    Steels                                                                             6  20   --   7.5                                                                              74   797  910                                                                              205                                                                              -70 179 preheating                            9  20   550  8.2                                                                              93   862  978                                                                              141                                                                              -50 33  preheating                                                                    not necessary                         10 20   550  7.9                                                                              94   1033                                                                              1154                                                                              159                                                                              -60 45  preheating                                                                    not necessary                         11 20   --   6.7                                                                              91   797  897                                                                              193                                                                              -75 152 preheating                                                                    not necessary                         12 20   --   7.3                                                                              95   1024                                                                              1180                                                                              176                                                                              -80 37  80                               __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________             chemical compositions (wt %, *ppm)                                                                                               P                 section                                                                            steel                                                                             C  Si  Mn P*  S* Ni  Cu Mo  Nb Ti  Al N*  others   value             __________________________________________________________________________    steel                                                                              1   0.060                                                                            0.29                                                                              1.96                                                                             120 20 0.42                                                                              0.98                                                                             0.42                                                                              0.040                                                                            0.012                                                                             0.030                                                                            33           2.29              of this                                                                            2   0.090                                                                            0.35                                                                              1.72                                                                             65  18 0.50                                                                              1.07                                                                             0.50                                                                              0.026                                                                            0.015                                                                             0.020                                                                            45  REM:                                                                                   2.318             invention                                                                          3   0.065                                                                            0.20                                                                              1.85                                                                             74  13 0.36                                                                              1.01                                                                             0.40                                                                              0.020                                                                            0.024                                                                             0.026                                                                            59  Cr: 0.65 2.55                   4   0.070                                                                            0.29                                                                              1.82                                                                             52  17 0.35                                                                              1.12                                                                             0.50                                                                              0.036                                                                            0.018                                                                             0.029                                                                            48           2.2                    5   0.071                                                                            0.25                                                                              1.71                                                                             128 18 0.45                                                                              1.03                                                                             0.42                                                                              0.045                                                                            0.020                                                                             0.035                                                                            37  V: 0.061 2.15                   6   0.069                                                                            0.05                                                                              1.92                                                                             84  16 0.39                                                                              0.92                                                                             0.49                                                                              0.035                                                                            0.018                                                                             0.018                                                                            39  V: 0.071 2.28                   7   0.078                                                                            0.24                                                                              1.84                                                                             65  10 0.48                                                                              1.15                                                                             0.48                                                                              0.040                                                                            0.019                                                                             0.002                                                                            30  Cr: 0.38, V:                                                                           2.740                  8   0.070                                                                            0.15                                                                              1.95                                                                             78  15 0.42                                                                              0.85                                                                             0.45                                                                              0.040                                                                            0.015                                                                             0.030                                                                            38  V: 0.08, Ca:                                                                           2.3020            Compar-                                                                            9   0.127                                                                            0.28                                                                              1.71                                                                             70  18 0.39                                                                              0.93                                                                             0.39                                                                              0.030                                                                            0.018                                                                             0.024                                                                            39           2.15              ative                                                                              10  0.080                                                                            0.26                                                                              2.17                                                                             160 18 0.40                                                                              1.02                                                                             0.40                                                                              0.037                                                                            0.017                                                                             0.026                                                                            32  Cr: 0.40 2.85              Steels                                                                             11  0.082                                                                            0.40                                                                              1.87                                                                             90  53 0.42                                                                              0.98                                                                             0.45                                                                              0.039                                                                            0.018                                                                             0.032                                                                            35           2.23              __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________                                                       field                                                                     HAZ weldability                         temper-                                                                           steel                                                                              austenite                    tough-                                                                            lowest                              ing plate                                                                              grain                                                                              M    M + B                                                                             mechanical properties                                                                        ness                                                                              preheating                          treat-                                                                            thickness                                                                          size dy                                                                            fraction                                                                           fraction                                                                          YS  TS  vE.sub.-40                                                                        vTrs                                                                             vE.sub.-20                                                                        temp.   re-                section                                                                            steel                                                                             ment                                                                              (mm) (μm)                                                                            (%)  (%) (MPa)                                                                             (MPa)                                                                             (J) (°C.)                                                                     (J) (°C.)                                                                          marks              __________________________________________________________________________    steel                                                                              1   ∘                                                                     15   5.2  65   98  835 940 224 -95                                                                              193 preheating                 of this                                            not necessary              inven-                                                                             1'  x   15   5.2  65   98  801 955 213 -85                                                                              193 preheating                 tion                                               not necessary                   2   ∘                                                                     20   7.4  90   97  918 1018                                                                              216 -85                                                                              177 preheating                                                                    not necessary                   3   ∘                                                                     22   8.0  74   99  840 1003                                                                              197 -90                                                                              159 preheating                                                                    not necessary                   3'  x   22   8.0  74   99  812 1023                                                                              200 -85                                                                              159 preheating                                                                    not necessary                   4   ∘                                                                     20   7.1  80   92  832 952 204 -90                                                                              182 preheating                                                                    not necessary                   5   ∘                                                                     22   6.8  82   91  846 970 214 -95                                                                              157 preheating                                                                    not necessary                   6   ∘                                                                     20   6.2  76   94  852 993 201 -85                                                                              220 preheating                                                                    not necessary                   6'  x   20   6.2  76   94  825 999 193 -80                                                                              220 preheating                                                                    not necessary                   7   ∘                                                                     25   6.4  85   100 906 1032                                                                              216 -90                                                                              227 preheating                                                                    not necessary                   8   ∘                                                                     30   5.9  70   91  850 990 226 -90                                                                              213 preheating                                                                    not necessary              Compar-                                                                            9   ∘                                                                     22   6.7  91   100 906 998  98 -80                                                                               66 80                         ative                                                                              10  ∘                                                                     24   6.1  85   91  947 1027                                                                               54 -75                                                                               38 125                        Steels                                                                             11  ∘                                                                     28   7.1  80   98  850 971 107 -80                                                                               58 preheating                                                                    not necessary              __________________________________________________________________________

We claim:
 1. A weldable high strength steel excellent in low temperaturetoughness; containing, in terms of percent by weight:C: 0.05 to 0.10%,Si: ≦0.6%, Mn: 1.7 to 2.5%, P: ≦0.015% S: ≦0.003% Ni: 0.1 to 1.0%, Mo:0.15 to 0.60%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.030%, Al: ≦0.06%, B: upto 0.0020% N: 0.001 to 0.006%, and the balance of Fe and unavoidableimpurities; and having a P value, defined by the following formula,within the range of 1.9 to 4.0; wherein the micro-structure of saidsteel contains at least 60%, in terms of a volume fraction, ofmartensite transformed from un-recrystallized austenite having anapparent mean austenite grain size (dγ) of not greater than 10 μm, andthe sum of said martensite fraction and a bainite fraction is at least90%:

    P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+(1+β)Mo-1+β

where β is 0 when B is less than 3 ppm and β is 1 when B is greater thanor equal to 3 ppm and wherein said steel has a tensile strength of atleast 950 MPa.
 2. A weldable high strength steel excellent in lowtemperature toughness, which contains at least one of the followingcomponents, in terms of percent by weight, in addition to said steelcompositions of claim 1:B: 0.0003 to 0.0020%, Cu: 0.1 to 1.2%, Cr: 0.1to 0.8%, and V: 0.01 to 0.10%.
 3. A weldable high strength steelexcellent in low temperature toughness, which contains at least one ofthe following components, in terms of percent by weight, in addition tosaid steel compositions of claims 1;Ca: 0.001 to 0.006%, REM: 0.001 to0.02%, and Mg: 0.001 to 0.006%.
 4. A weldable high strength steelexcellent in low temperature toughness, containing, in terms of percentby weight:C: 0.05 to 0.10%, Si: ≦0.6%, Mn: 1.7 to 2.5%, P: ≦0.015%, S:≦0.003%, Ni: 0.1 to 1.0%, Mo: 0.15 to 0.60%, Nb: 0.01 to 0.10%, Ti:0.005 to 0.030%, Al: ≦0.06%, N: 0.001 to 0.006%, B: 0.0003 to 0.0020%,and the balance of Fe and unavoidable impurities; and having a P value,defined by the following formula, within the range of 2.5 to 4.0;wherein the micro-structure of said steel contains at least 60%, interms of a volume fraction, of martensite transformed fromun-recrystallized austenite having an apparent mean austenite grain size(dγ) of not greater than 10 μm, and the sum of said martensite fractionand a bainite fraction is at least 90%:

    P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2MO,

and wherein said steel has a tensile strength of at least 950 MPa.
 5. Aweldable high strength steel excellent in low temperature toughness,which contains at least one of the following compositions, in terms ofpercent by weight, in addition to said steel compositions of claim 4:V:0.01 to 0.10%, Cu: 0.1 to 1.2%, and Cr: 0.1 to 0.8%.
 6. A weldable highstrength steel excellent in low temperature toughness, containing, interms of percent by weight:C: 0.05 to 0.10%, Si: <0.6%, Mn: 1.7 to 2.0%,P: <0.015%, S: <0.003%, Ni: 0.3 to 1.0%, Cu: 0.8 to 1.2%, Mo: 0.35 to0.50%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.030%, Al: ≦0,06%, N: 0.001 to0.006%, and the balance of Fe and unavoidable impurities; and having a Pvalue, defined by the following formula, within the range of 1.9 to 2.8;wherein the micro-structure of said steel contains at least 60%, interms of a volume fraction, of martensite transformed fromun-recrystallized austenite having an apparent mean austenite grain size(dγ) of not greater than 10 μm, and the sum of said martensite fractionand a bainite fraction is at least 90%:

    P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+Mo+V-1,

and wherein said steel has a tensile strength of at least 950 MPa.
 7. Aweldable high strength steel excellent in low temperature toughness,which contains at least one of the following components, in terms ofpercent by weight, in addition to said steel compositions according toclaim 6:V: 0.01 to 0.10%, and Cr:0.1 to 0.8%.
 8. A weldable highstrength steel excellent in low temperature toughness, which contains atleast one of the following components, in terms of percent by weight, inaddition to said steel compositions according to any of claim 4:Ca:0.001 to 0.006%, REM: 0.001 to 0.02%, and Mg: 0.001 to 0.006%.
 9. Aweldable high strength steel excellent in low temperature toughness,which contains at least one of the following components, in terms ofpercent by weight, in addition to said steel compositions according toclaim 6:Ca: 0.001 to 0.006%, REM: 0.001 to 0.02%, and Mg: 0.001 to0.006%.